Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density and high voltage have been commercialized and widely used.
A lithium metal oxide is used as a positive electrode active material of a lithium secondary battery, and a lithium metal, a lithium alloy, crystalline or amorphous carbon, or a carbon composite is used as a negative electrode active material. A current collector may be coated with the active material of appropriate thickness and length or the active material itself may be coated in the form of a film, and the resultant product is then wound or stacked with an insulating separator to prepare an electrode assembly. Thereafter, the electrode assembly is put into a can or a container similar thereto, and a secondary battery is then prepared by injecting an electrolyte solution.
Charge and discharge of the lithium secondary battery is performed while a process of intercalating and deintercalating lithium ions from a lithium metal oxide positive electrode into and out of a graphite negative electrode is repeated. In this case, since lithium is highly reactive, lithium reacts with the carbon electrode to form Li2CO3, LiO, or LiOH, and thus, a film may be formed on the surface of the negative electrode. The film is denoted as “solid electrolyte interface (SEI)”, wherein the SEI formed at an initial stage of charging may prevent a reaction of the lithium ions with the carbon negative electrode or other materials during charge and discharge. Also, the SEI may only pass the lithium ions by acting as an ion tunnel. The ion tunnel may prevent the collapse of a structure of the carbon negative electrode due to the co-intercalation of the carbon negative electrode and organic solvents of an electrolyte solution having a high molecular weight which solvates lithium ions and moves therewith.
Therefore, in order to improve high-temperature cycle characteristics and low-temperature output of the lithium secondary battery, a robust SEI must be formed on the negative electrode of the lithium secondary battery. When the SEI is once formed during the first charge, the SEI may prevent the reaction of the lithium ions with the negative electrode or other materials during repeated charge and discharge cycles caused by the subsequent use of the battery, and the SEI may act as an ion tunnel that only passes the lithium ions between the electrolyte solution and the negative electrode.
Various non-aqueous organic solvents have been used in electrolyte solutions. For example, propylene carbonate is mainly being used as a non-aqueous organic solvent, but the propylene carbonate may cause an irreversible decomposition reaction with a graphite material. In order to replace the propylene carbonate, binary and tertiary non-aqueous organic solvents based on ethylene carbonate (EC) have been used. However, since EC has a high melting point, its operating temperature may be limited and battery performance may be significantly reduced at a low temperature.